1. Field
The present disclosure relates to grating couplers capable of coupling light around a normal or nearly normal angle with a high efficiency. In particular, it relates to a resonantly enhanced grating coupler, for example a grating coupler with defect mode.
2. Related Art
Grating couplers are usually not very efficient when coupling around a right angle because the Bragg condition to extract normally out of the waveguide also corresponds to the second order bandgap of the grating. Most of the light gets back reflected into the grating instead of being coupled out.
D. Taillaert, H. Chong, P. I. Borel, L. H. Frandsen, R. M. De La Rue, and R. Baets, “A Compact Two-Dimensional Grating Coupler used as a Polarization Splitter”, IEEE Photon. Technol. Lett. 15, 1249-1251 (2003) describes a polarization splitting grating coupler. D. Taillaert, P. Bienstam, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides”, Opt. Lett. 29, 2749-2751 (2004) describes a reflector in the vertical direction to enhance efficiency. U.S. Pub. App. No. 2004/0156590 describes an optical waveguide grating coupler with curved grates. S. Miyanaga, and T. Asakura, “Intensity profile of outgoing beams from uniform and linearly tapered grating couplers”, Appl. Opt. 20, 688-695 (1981) and K. A. Bates, L. Li, R. L. Roncone, and J. J. Burke, “Gaussian beams from variable groove depth grating couplers in planar waveguides”, Appl. Opt. 32, 2112-2116 (1993) describe examples of varying the grate strength to tailor the emission profile. M. Miler, and M. Skalsky, “Chirped and Curved Grating Coupler Focusing Both Outgoing Beam and Guided Wave”, Opt. Commun. 33, 13 (1980) describe chirped gratings and focusing in the vertical direction. D. A. Louderback, G. W. Pickrell, M. A. Fish, J. J. Hindi, H. C. Lin, M. C. Simpson, T. J. Eustis, and P. S. Guilfoyle, “VCSELs with Monolithic Coupling to Internal In-Plane Waveguides using Integrated Diffraction Gratings”, Electron. Lett. 40, 1064-1065 (2004) describes an example of grating coupler placed inside a vertical cavity.
Developing a grating coupler to couple light into a waveguide through a ninety degrees bend presents a difficult design issue. One possibility is to place the horizontal waveguide and the grating coupler (etched into the waveguide) into a vertical cavity such as in the Louderback paper cited above. However, the approach in Louderback leads to complications. In particular, it necessitates a reflector both above and below the grating. For silicon-on-insulator based integrated optics fabricated with CMOS compatible processing steps there is no possibility to fabricate the top reflector (where “bottom” is where the substrate of the chip is located and “top” is above the chip).